Antenna system



April 11, 1939- J. F. MORRISON l 2,153,768

ANTENNA SYSTEM Filed May 9, 1936 F/G 2,4 Flc: 2B F/G D L [4 voLTAaE /5 gCURRENT F/G. 3A

.L VOLTAGE A T TORNEV Patented Apr. 11, 1939 UNITED STATES PATENTori-ICE ANTENNA SYSTEM Application May 9, 1936, Serial No. 78,751

11 Claims.

This invention relates to antennas and particularly to broadcastantennas.

As is well known, quarter wave or half wavelength tower or vertical masttype antennas are rapidly replacing wire type antennas, such as the Tantenna, in the broadcast eld. Possibly because of the assumptiongenerally entertained that the voltage distribution in a radiator issinusoidal and in the case of a half wave-length radiator is a maximumat both radiator terminals, the half wave-length tower antennas now inuse, as far as applicant is aware, are series-excited and baseinsulated. Also, as far as applicant is aware, the quarter wave-lengthantennas in use are base insulated, although the use of a groundedshunt-excited quarter wave-length antenna has been suggested. It nowappears that the voltage distribution in vertical radiators is complexrather than sinusoidal and that vertical antennas, regardless of length,may be satisfactorily connected directly to the earth and the costlyequipment ordinarily associated With broadcast antennas, such as baseinsulators and lightning discharge devices, eliminated.

It is one object of this invention to eiliciently radiate energyutilizing a minimum of equipment.

It is another object of this invention to reduce the cost of broadcastantenna systems designed to radiate a relatively large amount of power.

It is still a further object of this invention to improve the operationof shunt-excited grounded quarter wave-length Vertical antennas.

According to one embodiment of theinvention, a tower or vertical mastantenna having a wavelength of approximately .58 wave-length isconnected to the earth or to a ground radial screen through a path ofzero impedance. The transmitter is connected to an intermediate lpointon the tower by means of a line comprising a coaxial buriednon-radiating section adjacent the transmitter and a radiating sectionadjacent the tower. TheA radiating line section `is rigidly oradjustably associated with the tower, the connection or tapping pointbeing critically chosen a so that the resistance component of theimpedance terminating the coaxial line section equals the surgeimpedance of said coaxial section. A reactance of the distributed orlumped type is included in the radiating line to compensate or balancethe reactance of the coaxial line terminating impedance. The radiatingline section is inclined or angularly related to the tower, Whereby theradiation therefrom neutralizes the undesired radiation from the lowerantenna portion.

The invention will be more fully understood from the followingdescription taken in connection with the drawing on which `likereference numerals designate elements of similar function and on which:

(Cl. Z50-33) Fig. 1 illustrates one embodiment of the invention;

Figs. 2A, 2B, 2C and 2D are diagrams useful in explaining the operationof the system of Fig. 1;

Fig. 3A represents schematically the impedance terminating the coaxialline included in the system of Fig. 1;

Fig. 3B illustrates curves representing the various transmission lineterminating impedances obtainable in the system of Fig. 1; and

Fig. 4 represents a slightly different embodiment of the invention.

Referring to Fig. l, reference numeral I designates a fabricated towervertical mast antenna mounted directly on a concrete ground base 2 andconductively connected through a path of zero impedance comprisingstraps 3 to a ground screen 4. The antenna I may have any practicalheight H, but is preferably .58 wave-length high, and it may, ifdesired, be of the solid mast type. The antenna I may, if desired, besupported by means of guy wires 5 and associated insulators S. Referencenumeral 'l designates a radio frequency transmitter, and numeral 8designates a low impedance coaxial transmission line having an innerconductor 9 and an outer conductor I0 which is preferably buriedthroughout substantially its entire length. The ratio of the outsidediameter of the inner conductor to the inside diameter of the outerconductor is preferably in the neighborhood of 3.7. The inner conductor9 which may be tubular or solid, is connected through an adjustablecapacity II to an inclined radiating line conductor I2, the inclinedconductor I2 being adjustably or rigidly associated with an intermediateor tapping point I3 of the antenna I.

Reference is now made to Figs. 2A, 2B, 2C and 2D for the purpose ofexplaining the operation of the system of Fig. l. In Figs. 2A and 2B,reference numerals I4 and I5 designate free space antennas well known inthe art and having respectively a quarter and a half wave-length. Asindicated by the curves of these figures,sinusoidal voltage and currentdistributions are assumed in accordance with the theory heretoforeconsidered correct. The curves reveal that in the case of an energizedhalf wave-length radiator, the `voltage at each terminal or end is amaximum and in the case of an energized quarter wave-length radiator,the voltage has .a maximum value at one terminal and a zero value at theother terminal which may be considered the lower terminal. Consequently,it has heretofore been deemed necessary to insulate from the ground,which possesses a zero potential ordinarily, the lower high potentialterminal of vertical half Wave-length and aperiodic antennas; and thatlgrounding the lower zero potential terminal of the vertical quarterwave-length antenna would not affect its operation.

tenna portion, as indicated by curve i6.

As indicated by curve I6 of Fig. 2C, however, the grounded antenna I ofthe system of Fig. 1 is energized by transmitter "I and has an aperiodiclength or a length equal to approximately a half wave-length, thecurrent distribution and, therefore, the voltage distribution which isin quadrature with the current not being sinusoidal but complex. CurveI6 is based upon actual measurement and shows that the antenna currentadjacent the lower terminal of the approximately half wave-lengthradiator is a maximum and substantially a minimum at the tapping pointIii. It should be noted that the standing wave current in a sense passesthrough a minimum value and reverses at point i3, wherefore the currentrep-resented by the portion of curve I6 below the tapping point I3 issubstantially out of phase with the current represented by the portionof curve i8 above the tapping point I3. The coupling current in thelower antenna portion radiates energy and assists the radiation asproduced by the antenna current therein, whereby an intense field isestablished by the lower an- This lower antenna eld unless reducedwould, applicant believes, unfavorably affect the total radiation,elevate the direction of maximum antenna action and diminish the desiredground wave propagation while increasing the undesired sky wavepropagation. As will be explained presently, however, the conductor I'is deliberately inclined so that its radiation neutralizes in part, ifnot completely, the radiation from the lower portion of the antenna Icaused by the coupling current whereby the resultant radiation for thecomplete system comprising antenna I and conductor I2 corresponds to thecurrent distribution illustrated by curve Il. Curve I8 illustrates, byway of comparison, the current distribution of a .58 wave-length orapproximately half wavelength series-excited or base insulated antennaof the prior art; and it will be observed that the radiation effectsproduced by the grounded antenna of this invention and the costlyantenna of the prior art are substantially the same.

Referring to Fig. 2D, the eld cancellation effect secured by incliningconductor i2 will now be explained. Arrow i9 represents the direction offlow at a given instant of the current in con.- ductor I2 andcorresponds to the direction of the field initially radiated thereby.The eld has a horizontally polarized component 2@ and a verticallypolarized component 2l. The current supplied over conductor l2 to thetower divides .at the tapping point i3 and flows in opposite directionstherefrom. The resulting vertically polarized field radiated by theupper antenna portion is represented by arrow 22 and the oppositelydirected vertically polarized eld established by the lower antennaportion is represented by arrow 23, eld 23 being stronger than eld 22.As indicated by this vector diagram, eld 2i opposes eld 23 andsubstantially eliminates the effect produced by the coupling current.The vector diagram of Fig. 2D is applicable to vertical antennas excitedat an intermediate point and having any length including a quarterwave-length or a half wave-length since the coupling current field andeld of inclined conductor I2 oppose. It is believed to be apparent,therefore, that improved radiation may be obtained not only in the caseof shunt-excited aperiodic antennas but also in the case of shuntexcitedgrounded quarter wave-length antennas by supplying the energy theretoover a radiating conductor angularly related to said antenna. It may beadded that optimum results are secured by utilizing the inclined, ratherthan the vertical or horizontal position, for conductor I2, Fig. 2D,since in the case of the horizontal position no vertically polarizedwave cancellation is achieved and in the case of the substantiallyvertical position, a large portion of the antenna I is renderedineffective when the tapping point I3 is exceedingly high, as it mustbe, in order to obtain the proper terminating impedance for line 8.

Referring now to Figs. l and 3A, the manner of securing a properterminating impedance for line 8 will be explained. The load impedanceZS terminating the line 8 comprises the surge irnpedance Z0 between theline conductor I2 and the ground, the distributed impedance Za of theupper portion of the antenna and an impedance connected in shunt theretocomprising the impedance Ze of the lower antenna portion, whichimpedance is serially connected to the ground impedance Zg. As indicatedby Fig. 3B, by properly fixing or adjusting the dimensions h and/or d,indicated in Fig. 1, the proper value of resista-nce and reactance forterminating line 8 may be easily secured, the symbols r and :nassociated with the curves of Fig. 3B representing, respectively,resistance and reactance. To illustrate, a resistance of 60 ohms and areactance of 200 ohms may be obtained by making dimension h equal toapproximately .03 wave-length, and dimension d equal to approximately.O22 wave-length, or by adjusting h to equal .037 wave-lengthapproximately, and d .012 wavelength. n systems actually usedexperimentally, conductor I2 was inclined at an angle of between 40 to50 degrees, h .and d were each made equal to approximately feet, theantenna being approximately 500 feet high, in order to eliminate theeffect of the coupling current and to satisfactorily terminate line 8.

The terminating impedance Z1' of the inclined conductor is given by thefollowing equation:

Z|1(Z.9+Zg) T"Z..+Z.+Z (A) where Zs=the antenna impedance above the tapposition. Zc==the impedance of the antenna structure below the tapposition. Zg=a correction factor explained in the text. The sending endimpedance ZS of the inclined conductor is then by transmission linetheory for a loss-less line where :2W times the length of the inclinedconductor in wave-lengths (radians) Z=characteristic impedance of theinclined conductor 4D gies icgw-t- (C) where D=the average height of theinclined conductor above ground. t :the diameter of the inclinedconductor.

modication illustrated by Fig. 4 has the advantage that it is simplerand more economical.

Although the invention has been described in connection with certainembodiments, it is to be understood that it is not to be limited to suchembodiments and that other arrangements may be successfully employedWithout exceeding the scope of' the invention.

What is claimed is:

1. In combination, an antenna connected to a conducting surface througha path of zero impedance for energy of the operating frequency andhaving a length other than a quarter wavelength or an odd multiplethereof, and energizing means connected to an intermediatepoint of saidantenna.

2. In a radio broadcast system, a Vertical tower type antenna having aheight in the order of .4 to .6 wave-lengths and its lower terminalconnected to the earth through a path of zero impedance, a source ofradio frequency energy, and means connecting said source to anintermediate point in said antenna.

3. In a radio system, a half wave-length vertical radiating antennahaving its lower terminal connected to a ground screen through a pathof' zero impedance, substantially, and means for energizing said antennaat an intermediate point thereof.

4. In a radio system, a vertical radiating an" tenna having a lengthother than a quarter wave-length or an odd multiple thereof, saidantenna having its lower terminal connected to the earth through a pathor" zero impedance, substantially, and means connected to said antennafor causing the antenna portions above and below an intermediate pointthereof to establish dissimilarly phased radio fields and fordiminishing the intensity of the field established by the antennaportion below said point whereby a strong ground wave held is produced.

5. In a radio system, a vertical antenna having its lower terminalconnected directly to the earth for currents of the operating frequencyand a height other than a quarter wave-length or a multiple thereof, andmeans for causing current to flow in said antenna comprising a source ofenergy and a line comprising a conductor connecting said source and anintermediate point of said antenna whereby a strong field is establishedby said antenna.

6. In a radio system, a vertical antenna having its lower terminaldirectly connected to ground, a source of radio frequency energy, a linecomprising a conductor connecting said source to an intermediate pointof said antenna, and a rea'ctance included in said conductor and havinga value and characteristic equal and opposite, respectively, to theprimary reactance of the antenna portion below said connecting point.

'7. In a radio system, a vertical antenna having a length inthe order of.4i to .6 wave-lengths, said antenna having its lower terminal directlygrounded, a source of radio energy frequency and a conductor connectingsaid source to an intermediate Vpoint of said antenna and having aninclined radiating portion adjacent said antenna.

8. I a radio system, a vertical tower antenna, a source of energy, aline connecting said source and antenna, said line comprising anon-radiating coaxial section adjacent the source and a radiatingsection adjacent the antenna, said radiating section being inclined,whereby the field radiated by said section includes a verticallypolarized component oppositely directed or phased with respect to thevertically polarized component radiated by the antenna portion belowsaid connecting point and a maximum field is radiated horizontally.

9. In combination, a vertical transmitting antenna having a height otherthan a quarter wavelength and its lower terminal directly grounded, asource of radio frequency energy having one terminal directly grounded,and a coaxial transmission line having its cuter conductor grounded andits inner conductor connected between the remaining terminal of thesource and a point on said antenna at which the impedance terminatingsaid line equals approximately the surge impedance of the line.

10. In a radio system, a vertical grounded antenna, a source of radiofrequency energy, a buried coaxial line, connected to the terminals ofsaid source, an inclined linear conductor connected to an intermediatepoint of said antenna, a capacitive reactance connected between saidinclined conductor and the inner conductor of said line, the length andinclination of said conductor being critically chosen and such as toprevent reiiection on said line.

l1. A system in accordance with claim 10, said capacitive reactancecomprising a plurality of spaced linear conductors.

JOHN F. MORRISON.

